Antibiotic susceptibility testing has for many years been used as a means for identifying particular groups or species of micro-organisms, or for identifying an antibiotic type or dose level most appropriate for dealing with a particular clinical infection.
Usually, six or eight discs, carrying different types of antibiotic or the same type of antibiotic but in different concentrations, are placed in a circular array on a petri dish. The dish contains a layer of growth medium, such as agar gel, to which a material containing a micro-organism to be analysed is applied.
The antibiotic diffuses out of each disc into the surrounding growth medium and establishes a radial concentration gradient around the disc. The diameter of the zone of inhibited micro-organism growth around the disc is indicative of the relative susceptibility of the micro-organisms to the antibiotic on that disc. The detailed morphology of the zone can also provide information on the species or genus of micro-organism presence.
There have been various proposals for automating the analysis of the contents of the petri dishes, especially the inhibition zones, involving the use of an overhead video camera connected to a data processing device.
The petri dishes used in such tests are commonly provided with identification markings. For example, in hospitals, the patient from whom the micro-organism in a petri dish has been extracted is identified by means of a bar code applied to the side of the petri dish.
Since the sides of the petri dishes are vertical, it is difficult if not impossible for the bar codes to be read by an overhead camera of an automatic analysis system.